What is Plant hormone?
- Plant hormones are also termed as phytohormones (named by Thieman), growth factors, growth regulators, growth substances etc.
- Phytohormone is an organic substance, naturally produced in higher plants that regulate plant physiological process such as affecting growth and other functions remote from its place of production and active in very minute amounts.
- They can be either natural or synthetic, stimulatory or inhibitory in nature.
- They act at a distance from the place where they are formed.
- Three types of phytohormones are mostly recognized. They are:
- Auxin
- Gibberellin
- Cytokinin
1. Auxin:
- An auxin is an organic compound responsible for promoting the growth of plants along the longitudinal axis when applied in low concentrations to shoots of the plants.
- Auxin is specifically concerned with cell enlargement or the growth of the shoots.
- Auxin is identical to Indole 3-Acetic Acid (C10H9O2N, IAA), i.e. natural true auxin.
- The precursor of Indole 3-Acetic Acid is tryptophan and zinc play a role in its biosynthesis.
- Auxin exhibits polar movement i.e.
- Basipetal movement (from apex to base) in case of shoots.
- Acropetal movement (from root tip to shoot) in case of roots.
- Bioassay test: Bioassay is termed as the functional test of substance in living plants.
- The common bioassay tests of auxin are Avena coleoptile test and root growth inhibition test.
What are the physiological roles of auxin in plants?
- Besides the cell enlargement and growth, auxin (both natural and synthetic) are responsible for various other growth processes. They are:
- Cell elongation:
- The cell elongation occurs only in the presence of auxin and the rate of elongation is directly proportional to the amount of auxin supplied, given no other factors are limiting.
- However, relatively high concentrations of auxin show inhibitory effects on this phase of growth.
- Auxin promotes the elongation of roots at its low concentrations, the growth of roots is retarded at higher concentrations.
- Flowers need higher concentration of auxin for their growth.
- Auxin also induces the elongation of coleoptiles and stems by cell enlargement.
- Auxins are responsible for the elongation of petiole, mid rib and major lateral veins of the leaves.
- Hence, adenine aids in enlargement in detached leaves of radish and pea. Similarly, coumarin has been shown to promote expansion of leaves in some plants.
- Cambial activity:
- During the spring season, the trees manifest growth by developing buds that later on open and elongation of young stems take place.
- Auxin activates this resumed growth by cambial cells
- The growth moves basipetally in the stems from developing buds.
- Callus formation and galls:
- The auxins activate cell division.
- When 1% IAA in lanolin paste is applied to a de-bladed petiole of a bean plant, prolific division of parenchyma cells occurs.
- A swelling or callus tissue is formed at the point of application of auxin.
- The amount of callus tissue formed is directly proportional to the concentration of IAA applied.
- Apical dominance:
- Apical dominance is the major function of auxin.
- The growth of lateral buds is suppressed until apical bud is present in the plants.
- This inhibitory effect of terminal bud upon the growth of lateral buds is termed as apical dominance.
- Skoog and Thimann (1934) first reported the relation of apical dominance
with the auxin supply. - When agar block containing auxin b or IAA was kept on the decapitated shoot of broad bean (Vicia faba), the lateral buds, as might be expected, resulted in the usual suppression of growth.
- But when the same decapitated shoot was re-headed with an agar block containing no auxin, these lateral buds resumed growth.
- When NAA was used as auxin in field-grown tobacco plants, similar results were obtained.
- Evidence of apical dominance has been practically used in solving the potato storage problem.
- Potatoes, stored for some time, sprout and become sweet in taste, causing the grower to lose financially as its consumers hate the sweet taste.
- But by inhibiting the growth of buds or ‘eyes’ by spraying potatoes with auxins such as indole butyric acid ( IBA) and NAA, sprouting (or in other words, prolonging dormancy) can stop sprouting; the effect lasts for as long as 3 years.
- Rooting of stem cuttings (Formation of adventitious roots):
- It is a common observation that when the lower end is dipped in an acceptable rooting medium, the appearance of buds on a cutting promotes the growth of roots.
- In accelerating root formation, developing buds are efficient.
- The initiation of roots on the cuttings are often favoured by young leaves.
- These findings contributed to the idea that the auxins synthesized in the buds and young leaves favour the root formation and are later translocated to the basal part of the cut.
- IAA, NAA, 2,4-D, naphthalene acetamide (NAd) etc are the auxins most widely used for this function.
- Auxin-induced rooting is also of considerable horticultural benefit as it allows cuttings to propagate those plants.
- Delay (or inhibition) of abscission of leaves:
- By adding auxins on the surface of the lamina or on the cut surface of a debladed petiole, abscission of the leaves may be delayed or hindered.
- Laibach (1933), who demonstrated that the extract of orchid pollinia is capable of preventing leaf dropping, first noted the regulating actions of auxins on abscission.
- Since then, sufficient work in this direction has been carried out.
- The delaying effect of IAA on the abscission of different plant organs has been shown conclusively by Addicott and Lynch (1955).
- As for the abscission process, it has been proposed that the basipetal migration of a hormone from the blade to the base of the petiole retards the leaf drop.
- Leaf blade removal removes the hormone supply to the abscission zone and thus causes the drop of the leaf.
- Flowering:
- Auxins play role in modifying flowering by following ways:
- Producing early flowering
- Inducing flowering
- Preventing or delaying flowering
- Fruiting:
- Auxins play significant role in fruiting by altering it in one of the following ways :
- Fruit setting:
- The changes in the ovary leading to the development of the fruit is termed as fruit set.
- These changes are generally induced after pollination and fertilization.
- The development of fruit without fertilization is termed as parthenocarpy.
- It is a common characteristics in plants and hence occurs frequently.
- The parthenocarpy can be induced artificially by the aid of auxin.
- For example, Yasuda (1934) demonstrated it by application of pollen extracts to cucumber flowers.
- It was also observed that ovaries of many plants (orange, lemon, grape, banana, tomato etc.) could be induced to develop into seedless fruits by application of IAA in lanolin paste to their stigmas.
- The various other auxins used for parthenocarpy are IPA, IBA, α-NAA, phenoxyacetic acid (POA), α- naphthoxyacetic acid (NOA) etc.
- Fruit thinning:
- The trees, in many cases, bear a large number of fruits.
- It leads to the inability of the trees to grow an average number of new flower buds.
- Therefore, such trees must grow fruit either in alternate years (alternate bearing) or if yearly, the number of fruits is significantly reduced.
- Clearly, these trees need thinning.
- For the first time, fruit thinning was achieved in apples when naphthalene acetic acid added to flowers failed to set the fruits and actually caused a decrease in the set of fruits.
- It is interesting to note that the only effective auxin that induces fruit thinning seems to be naphthalene acetic acid.
- However, a-2,4,5-trichlorophenoxyacetic acid for thinning of pears and p-chlorophenoxyacetic acid for thinning of grapes are other examples of auxins used for fruit thinning.
- Control of premature fruit dropping:
- The development of an abscission sheet causes the falling of unripe fruits in many fruit trees causes significant losses in yield to the gardeners.
- In several cases, such as apples, the problem has now been successfully overcome by the application of auxins.
- Auxins prevent the formation of the abscission layer and thus check the drop of the fruits before harvesting.
- With 2,4-D and 2,4,5-trichlorophenoxyacetic acid as auxins, regulation has also been induced in citrus fruits (like oranges and lemons).
- Improving the quality of fruits:
- The different processes such as colouring, softening, sweetening and ripening are all involved in improving the fruit ‘s quality.
- In apples, where the use of 2,4, 5-trichlorophenoxyacetic acid has significantly increased red pigments, the auxin effects on fruit colouration are most noticeable.
- 2,4-D accelerated the ripening process when added to bananas as the auxin stimulates the conversion of starch into sugars.
- Sugar accumulation has been reported in sugarcane by injecting 2,4-D, IBA or maleic hydrazide.
- Increase in respiration:
- Auxins enhances the respiration process. It was first identified by James Bonner in 1953.
- A direct relation between growth due to auxin treatment and rate of respiration has been found i.e., greater the growth, higher is the respiration.
- Auxins are used to control the growth of weeds in the crop fields.
- 2,4-D is sprayed for the weeds in the crop fields that acts as weed killer.
- Graminaceous weeds are destroyed by 2,4-dichloropropionic acid.
- Increased resistance to frost damage:
- When parsnip is treated by 2,4,5-T, the tops resist damage by frost.
- In apricot fruits, the application of 2,4,5-T before the onset of frost caused less damage than the untreated fruits.
- Great weapon of war:
- When auxins are applied in higher concentrations on enemy crop fields by means of air, it causes devastation of land and form the basis for biological warfare.
2. Gibberellins:
- E. Kurosawa, first discovered gibberellin from a fungus called Gibberella fujikoroi in the year 1926.
- A gibberellin is abbreviated as GA, for gibberellic acid.
- Gibberellin may be referred as a compound which is active in gibberellin bioassays and possesses a gibbane ring skeleton.
- However, there are other compounds (like kaurene) that are active in some of the assays but lack a gibbane ring. Such compounds have been termed gibberellin-like rather than gibberellins.
- Brian isolated pure sample of a single gibberellin and termed as gibberellic acid.
- The structure for gibberellic acid was given by Cross et al in 1961.
- More than 100 types of gibberellin are known, among them GA3 is most common.
- Gas are common in all groups of plants, however it acts as a flowering hormone in angiosperms only.
- All gibberellin possess gibbane ring. Gibbane ring consists of 4 isoprene units (hence, 2 terpenes, di-terpenes).
- The 5 carbon compound isopentenyl pyrophosphate is the precursor of gibberellin.
- Bioassay test: Gibberellins are synthesized via the mevalonic acid (MVA) pathway.
- The biosynthesis of GA3 from MVA takes place by 18 or more steps or intermediates and about 15 associated compounds.
What are major Physiological effects of Gibberellin in plants?
- Genetic dwarfism:
- In some plants, the mutation of a single gene causes dwarfism.
- Such individuals are termed as ‘single gene dwarfs’.
- In these plants dwarfism is due to shortening of internodes rather than reduction in number of internodes.
- The use of gibberellins on such dwarfs causes them to elongate to the point of being indistinguishable from common tall plants.
- Hence, gibberellin A3 treatment has been used to overcome genetic dwarfism successfully in many single gene dwarf mutants like Pisum sativum, Vicia faba and Phaseolus multiflorus.
- Gibberellin also induces leaf expansion.
- Bolting and flowering:
- Rosette plants are marked by the prolific growth of leaves and the delayed growth of internodes.
- But there is striking elongation in the internode before the reproductive process, so that the plant reaches 5 to 6 times the initial height.
- The treatment of these ‘rosette’ plants with gibberellins stimulates bolting (or shoot elongation) and flowering under conditions that would normally preserve the rosette shape.
- It is also possible to distinguish shoot elongation from flowering by controlling the amount of gibberellin applied.
- The plant can bolt but not flower with low gibberellin dosages.
- GA3 hastens the flowering and flower yield in many plants such as Coriandrum sativum (coriander).
- Gibberellin controls flowering in long day plants.
- Light inhibited stem growth:
- The dark-grown plants showed better stem growth in comparison to light grown plants.
- This inhibitory effect of light on stem elongation could be reversed by the use of gibberellins in plants as such Pisum sativum.
- This clearly indicates that the gibberellin is the limiting factor in stem elongation.
- Parthenocarpy:
- Gibberellins induce parthenocarpy more efficiently than auxins.
- It has been found in plants such as Cucumis sativa (cucumber), Zepyranthes sp., Solanum melongena (brinjal).
- Breaking dormancy of seeds:
- In the light sensitive seeds (lettuce, tobacco), the germination is retarded in dark.
- The application of GA3 allows the germination of seeds in dark as well.
- Breaking dormancy of buds:
- Because of very low temperature, the buds produced in winter stays dormant till the next spring in temperate areas.
- Gibberellin treatment overcomes the dormancy in such cases and replaces the light requirement for breaking dormancy.
- It breaks dormancy in potato tubers as well.
- Role in abscission:
- The abscission has been enhanced in explants of bean and Coleus by the GA3 treatments.
- Stimulation of enzyme activity in cereal endosperm:
- It was demonstrated that the exogenous application of gibberellins stimulated amylase activity in isolated barley endosperm.
- It was also found that the treatment of isolated aleurone layer of endosperm with GA could release enzymes, amylase and proteinase.
- Sex expression:
- Gibberellins show the capability to alter the sex of the flowers.
- It promotes the production of male flowers in cucurbitis, Cannabis etc.
- Also, the antheridia have been induced to form in many fern gametophyte
- Juvenility:
- Most of the plants manifests two different stages of growth i.e. a juvenile stage and an adult stage.
- The application of gibberellin helps to determine if a specific part of plant is juvenile or not.
3. Cytokinins:
- Cytokinins are alos named as kinetins because of their absolute power to enhance cell division in the presence of an auxin.
- First naturally occurring cytokinin was recognized from young maize grain by Letham and termed as zeatin.
- Fox (1969) has defined cytokinins as chemicals composed of one hydrophilic adenine group of high specificity and one lipophilic group without specificity.
- Chemically, kinetin (C10H9ON5) is 6-furfurylaminopurine.
- Cytokinins occur in higher plants, diatoms, red and brown algae, mosses.
- These occur widely in embryo sac, roots during seedling stage, flowers, developing fruits, cambial tissue and endosperm.
- The richest source of kinins are fruits and endosperm.
- Bioassay test: Callus pith cell division, chlorophyll retention test, soybean and radish cotyledon cell division are the main bioassay tests.
What are Physiological roles of cytokinin in plants?:
- Cell division:
- In addition to auxins, the kinins are required in right ratio of concentrations for the enormous growth response .
- When mixture of auxin and cytokinin is added to unspecialized cells, their differentiation begins.
- A high cytokinin to auxin ratio results the formation of shoots, buds and leaves while a low cytokinin to auxin ratio causes root formation.
- This invitro culture methods allows the rapid production of large number of plants in a small space.
- Cell elongation:
- Kinetin also enhances cell elongation.
- It has been demonstrated in tobacco pith cultures, tobacco roots and bean leaf tissues.
- Root growth:
- Kinetin is responsible for both the stimulation as well as inhibition of root development.
- When kinetin was applied along with IAA, the root initiation and development in stem callus cultures was stimulated.
- In lupin seedlings, Kinetins induced increase in dry weight and elongation of the roots.
- Shoot growth:
- When the balance of IAA and kinetin is maintained, the callus tissue of tobacco can be kept in an undifferentiated state for a long time.
- When the amount of kinetin is increased, the development of leafy shoots begins.
- Organogenesis:
- Organogenesis is resulted by cytokinins in several tissue cultures.
- By changing the relative concentrations of kinetins and auxins, the tobacco pith callus can be directed to develop either buds or roots.
- High kinetin and low auxin contents causes the production of buds.
- The roots appear in pith in reverse condition, i.e. high auxin and low kinetin contents.
- In leaf segments of various plants such as Saintpaulia ionantha, Bryophyllum sp and Begonia sp., the kinins stimulate the production of buds.
- In addition to the root and shoot differentiation, the cytokinins also bring about other morphogenetic responses.
- These are :
(a) maturation of proplastids into plastids
(b) differentiation of tracheids
(c) induction of parthenocarpy
(d) induction of flowering
- Counteraction of apical dominance:
- Cytokinins are powerful promoters of lateral bud growth.
- When the culture medium consists of IAA, the growth of lateral buds is inhibited, but the addition of kinetin along with IAA stimulates the growth of lateral buds.
- Breaking dormancy of seeds:
- Cytokinins show effective role in breaking seed dormancy in lettuce, tobacco, white clover and carpet grass.
- In such cases, the site of cytokinin action is cotyledon.
- The seeds of parasites such as Striga asiatica need the host plant for germination. But when treated with kinetin, the seeds germinate even in the absence of their host.
- Delay of senescence (Richmond-Lang effect):
- The ageing of leaves along with the loss of chlorophyll and the breakdown of proteins is termed as senescence.
- Richmond and Lang demonstrated that the senescence in the detached leaves of Xanthium could be postponed for several days by kinetin treatment.
- This effect of kinetin in retarding senescence is termed as Richmond-Lang effect.
- Role in abscission:
- Depending on the site of application, cytokinins can either accelerate or retard the process of abscission in leaf petioles.
- It is the common property of cytokinin.
- Effects on cotyledons:
- Cytokinins enhances cellular division and expansion in cotyledons.
- Cytokinins increase the concentration of sugars in cells resulting in endosmosis that causes the expansion of cytokinin treated cells in cotyledons.